Method for storing energy in the form of thermal energy by means of high-temperature accumulators

ABSTRACT

The invention relates to a method for actuating nonsystem-connected vehicles by means of high-temperature accumulators and for the operation of stationary energy storage using material with high evaporation enthalpy as storage media for high-temperature heat. Said high-temperature heat is generated by means of current transfer and can be stored for a sufficient length of time by means of super isolation and high-temperature resistant carbon materials. On demand, the stored high-temperature heat can be converted directly into electric operating energy, specifically pressure energy, for actuating hydraulic motors by means of a thermionic generator or a Stirling motor. The average capacity of high-temperature accumulators is 10 kWh/kg greater than that of internal-combustion machines and is 100 times greater than the present highest capacity electrochemical working accumulators, thus enabling the operation of all fuel operated vehicles to be more comfortable, more affordable and cleaner in the future due to high-temperature accumulators.

The invention relates to a method in which thermal energy is stored inrapidly chargeable high temperature accumulators which can be converteddirectly via a thermionic generator into electrical drive energy or viaa Sterling engine directly into pressure energy for the driving ofhydraulic motors.

High temperature accumulators which operate on an electrochemical basisare known like the sodium/sulphur high temperature system of ABB whichhas the highest output at the present time with 0.08-0.1 kWh/kg and thesodium/nickel chloride high temperature system of AEG Anglo Batteries,under the product name ‘Zebra Battery’ with 0.08 kWh/kg.

These so-called high temperature systems require operating temperaturesof about 300° C. at which the oxide ceramic electrolyte is conductive toions and permits the electrode reactions resulting from the currentsupplied. To maintain the operating temperature continuous heating isrequired, otherwise the accumulator fails and discharges within a fewdays.

Apart from the danger of accidents, the high cost, the high weight, thelow operational range and the high charge times are negative factorswhich make these systems noncompetitive with internal combustionengines. The Zebra Battery which is ripe for reproduction, is still 500DM/kWh even when produced in quantity and at drive powers like those ofa gasoline engine with a storage capacity of 50 kWh, can have a weightof 600 kg and an accumulator cost of 25,000 German marks.

A significant problem with such systems is the reactive and corrosivemelts which can, in the case of a serious accident, ignite upon contactwith water and air. Liquid sodium reacts explosively in combination withwater. The alkali sulfur melt is highly corrosive to steel and causesbreakdown of the battery housing even in the case of hairline cracks andignites upper contact with air.

In combination with water highly poisonous hydrogen sulfide can result.For these reasons, ABB has limited work on the sodium/sulfur battery.

By contrast with these electrochemically operating high temperatureaccumulators which by comparison with the method of the invention can beconsidered low temperature accumulators, the high temperatureaccumulator system of the invention operates with a many times highertemperature which, depending upon the respective accumulator materialused, can be higher by a factor of 10.

The object of the high temperature accumulator process according to theinvention is to operate vehicles of all kinds, whether land vehicles,water vehicles, air vehicles or space vehicles, independent fromelectrical networks in as comfortable a manner or more comfortably thanwith internal combustion engines without the drawbacks of the latter,like release of toxic materials, high noise level and high maintenancerequirements. A further object is the provision of an immediatelyavailable stationary energy store independent of power networks.

These objects are attained in that the highest possible quantity of heatper unit weight or unit space is stored in a fuel free manner in a hightemperature accumulator according to the invention, is conserved overthe longest time possible and upon demand is discharged again with thehighest possible efficiency as drive energy.

The basic material for attaining the objects is graphite which serves asthe carrier, storage pressure retentive, reinforcing and hightemperature insulating material and is used simultaneously also for heatand current conduction and for resistance heating. Carbon is thematerial with the highest temperature resistance of about 3500° C. inthe solid phase, is not wetted by molten metal and is thus used as acarrier material and as a material for heating up the high temperatureaccumulator but also as the heat storage material itself and is thusoptimal for the high temperature accumulator.

Carbon can be worked and shaped as required. Carbon is a high capacityproduct as to its properties like thermal conductivity, electricalconductivity, density, compressive strength and tensile strength and canbe produced with accurate dimensions in production processes. Theproduct palette in carbon ranges from large blocks through fibers tocomposite materials, foils and insulating felts. From this availabilityof products alone it is possible to produce a carbon accumulator with amaximum capacity of 25 kWh/l.

The method according to the invention, uses apart from graphite as astorage material in this sense, other elements which have greatercapacities or compounds of elements with higher melting and evaporationenthalpies and utilizes the fact that between the melting point andevaporation point a four times higher energy quantity can be taken upand stored than is necessary for the heating up of the material to amelting point. By evaporation of the storage material, a ten timesgreater energy quantity can thus be taken up whereby an optimum energyquantity per kWh/kg is storable therewith in the high temperatureaccumulator high pressure process which can optimally lie at 20 kWh/kgor 50 kWh/l.

Advantageously, nontoxic high temperature storage materials whichsublime and directly pass from the solid into the gaseous state. Thus inthe case of an accident, no melt is provided that run out, can causefire or can trigger toxic reactions. Of significance also are thethermal conductivity and the electrical resistance which ensure a rapidtransition from electric current to high temperature heat.

With respect to the properties of the high temperature storage materialsand the temperature levels which are used the high temperatureaccumulators of the invention can be divided into high pressure unitsand low pressure units whereby the state of aggregation of the storagematerials can be solid, liquid, gaseous, solid-liquid, liquid-gaseousand solid-gaseous.

For the storage of the high temperature heat the invention utilizes thefact that the efficiency of a thermionic generator or a Sterling engineincreases with increasing temperature difference between the cold andhot regions. In this manner energy losses in the transmission fromstored thermal energy to drive energy are held small. By comparison withan internal combustion engine which operates only with a fuel efficiencyof 15%, thermoelectric generators and sterling engines operate with anefficiency in excess of 50% with high temperature heat and cooling toambient temperature.

The overall efficiency and the cost/use balance by comparison totransport systems with internal combustion engines is increased furtherby the direct drive via current or hydraulic motors since the old highcost and weight intensive mechanical systems of conventional vehicles,like internal combustion engines exhaust systems, transmissions,universal joint shafts, starters, starting batteries, brake systems,fuel pumps, tanks and the maintenance and replacement thereof areeliminated. Motor oils, transmission oils, fuel and coolants aresuperfluous.

The storage material of the high temperature accumulators according tothe invention comprise, apart from graphite, preferably elements fromthe groups of beryllium, boron, lithium, silicon and their borides,carbides and nitrides, most of which sublimate advantageously without aliquid phase. For use in a liquid phase, metals are also suitable ashigh temperature storage materials like, for example copper or vanadiumat temperature ranges up to 3500° C. The energy densities of theaforedescribed high temperature storage material lie between 10 and 50kWh/l which may be higher by a factor of 100 than the highest capacityelectrochemically operating high temperature accumulators.

From the point of view of volume the storage of Be₂C and B₄C can be 35kWh/liter at about 2500° C.

The energy used for heating up the storage material with electriccurrent is converted in the transition from the liquid state into thesolid state or from the gaseous into the solid state of aggregationagain and is again fully available and can be transformed by generatorsto drive energy.

Advantageously, the charging with electric current is carried out bypassing the current directly through the storage material and thusproducing the high temperature heat by means of internal resistanceheating. For an external heating of the storage material which ispermissible by inductive heating, a removal or exchange of the storagematerial is required by an automated apparatus. The maintenance of thehigh temperatures for high temperature storage durations is ensured by asuper insulation which is achieved in accordance with the invention byintroducing the storage material under a protective gas (nitrogen,argon) into a pressure-tight and gas-tight hollow graphite cylinder(hollow graphite body) and directly heating it by current flow and/orindirectly heating it by thermal radiation from the graphite cylinderheated up by the electric current flow therethrough. The graphitecylinder, in the high pressure process must be able to retain theincreased gas pressure of the sublimated storage material. To ensurethis, either storage materials are selected with a reduced gas pressureat the sublimation point or the graphite cylinder is reinforced withcarbon graphite fibers to retain the higher pressures.

For optimum thermal insulation, the graphite hollow body (carrier of thehigh temperature storage material) can be insulated by alternatinggraphite hard felt layers of reduced thermal conductivity and made gastight and protective gas layers, whereby the inner graphite surfaces areprovided with radiation reflectors.

A 40 mm thick SIGRATHERM hard felt layer forms a barrier underprotective gas with a heat source of 2000° C. to thermal radiation of 10kW/m² to limit the temperature on the opposite side thereof to 400° C.The thermal conductivity of the hard felt layer at 2000° C. amounted toabout 1 W/K m and falls at 400° C. to 0.2 W/K m. At these temperatures,inexpensive rockwool felts with a thermal conductivity of a factor often less can be used for insulation. After insulation to ambienttemperature the high temperature accumulator can be provided with astable gas-tight shell which can be equipped with devices for automatedenergy takeup and energy discharge.

To insulate a hot cylinder of a diameter of 250 mm at a temperature of2500° C. to room temperature, only one insulation layer sequence of theaforedescribed construction with a thickness of only about 250 mm isrequired. It thus is apparent how high energy capacities can be taken upwith high temperature accumulators in small spaces.

An example of a low pressure high temperature accumulator according tothe invention utilizes BN (boron nitride) which sublimates as a storagematerial at 2400° C. and has an average heat storage capacity makes thisclear.

A cylinder of carbon with dimensions of 30×30 cm and a volumetriccapacity of about 20 l stores an energy quantity of about 100 kWh at3000° C. If one calculates a thermal insulation also with a layerthickness of about 30 cm, the carbon high temperature accumulatorincluding its gas tight outer shell will have overall dimensions ofabout 100×100 cm at 100 kg total weight. A high temperature accumulatorof these dimensions plus a generator, installed in the Sero-Emission-Carrequires only the space occupied by the superfluous internal combustionengine with its drive systems or can be provided in a flatter form inthe bottom plate. The zero emission auto then only requires anelectronic control, four wheel-drive motors which can be used asgenerators upon braking and can feed the brake energy in the form ofthermal energy again into the high-temperature accumulator or in thecase of hydraulic motors can charge a pressure accumulator.

With an efficiency of 40 to 50% and a 100 kWh storage capacity, a zeroemission automobile can travel with a high temperature accumulator 500to 600 km with a charging time of five minutes. At current costs, evenwith present day expensive household current of 0.25 German Mark/kWh,500 km of travel with the high temperature accumulator costs only about25 German Marks, five times less than the fuel cost of a present daymidsize automobile. With the aforementioned superinsulation, a standtime of five months can be expected before the high temperatureaccumulator is depleted as a result of heat loss. The life of anoptimized high temperature accumulator of the invention with thecapacity of 100 kWh/10 l with a total weight of 100 kg is equal to thelife of the vehicle.

The object of the high temperature accumulators according to theinvention to replace internal combustion engines for land vehicles, airvehicles and water vehicles and to provide emission free transport inall regions of the transport sector free from breakdown, is ensured withthe given features of the high temperature accumulators.

Power profile of an average high temperature accumulator. Power/height =100 kWh/100 kg Power/cost = 1 kWh/50 German Marks High temperatureaccumulator cost = 5000 German Marks Range = 500 km Life = 25 yearsCharging time = 5 minutes Current cost/km = .05 Mark/km Current cost = 5Mark/100 km

What is claimed is:
 1. A high temperature accumulator for the storage ofhigh temperature thermal energy, comprising: a hollow graphite bodyprovided with a means for heating contents thereof; and a heatable hightemperature storage material received in said graphite body andconvertible by heating from one physical state to another.
 2. The hightemperature accumulator defined in claim 1 wherein said materialincludes a material selected from the group which consists of graphite,beryllium, boron, lithium, silicon and the borides, carbides andnitrides thereof.
 3. The high temperature accumulator defined in claim 1wherein said material includes a metal selected from the group whichconsists of copper and vanadium.
 4. The high temperature accumulatordefined in claim 1 wherein said material is in the form of a solid orliquid and is transformable by heating into a liquid or gas.
 5. The hightemperature accumulator defined in claim 1 wherein said material isstored in said body under a protective gas.
 6. The high temperatureaccumulator defined in claim 1 wherein said means for heating includesmeans for passing an electric current directly through said body orthrough said material.
 7. The high temperature accumulator defined inclaim 1 wherein said material is transformable by heating from a solidphase to a gas phase by sublimation without passing through a liquidphase.
 8. The high temperature accumulator defined in claim 1 whereinsaid material has a high vaporization enthalpy.
 9. The high temperatureaccumulator defined in claim 1 wherein said body is a graphite bodyreinforced with carbon fiber fabric.
 10. The high temperatureaccumulator defined in claim 1, further comprising layers of graphitefelt alternating with protective gas layers surrounding said body andinsulating same said graphite felt layers having inwardly facingsurfaces with reflective coatings.
 11. The high temperature accumulatordefined in claim 1, further comprising a gas tight shell surroundingsaid body and provided with devices for automated energy reception anddischarge.
 12. The high temperature accumulator defined in claim 1wherein the energy capacity of said material is 10 to 50 kWh per liter.13. The high temperature accumulator defined in claim 1 wherein saidmaterial consists at least in part of graphite and said accumulator isprovided with thermal insulation, pressurization material and electriccurrent insulation all of graphite.
 14. The high temperature accumulatordefined in claim 1, further comprising a generator for recoveringthermal energy from the accumulator.
 15. The high temperatureaccumulator defined in claim 14 wherein said generator is a thermionicgenerator or a Sterling engine.
 16. The high temperature accumulatordefined in claim 1 wherein said accumulator is configured as an energysource for a vehicle.
 17. The high temperature accumulator defined inclaim 1 wherein said accumulator is configured as a stationary heat andcurrent supplying unit.
 18. A method of storing and regenerating energywhich comprises: providing a high temperature accumulator whichcomprises a hollow graphite body provided with a means for heatingcontents thereof and a heatable high temperature storage materialreceived in said graphite body and convertible by heating from onephysical state to another; supplying thermal energy to said material tochange a state thereof at least in part; and recovering energy from saidmaterial by driving a generator with energy therefrom.
 19. The methoddefined in claim 18 wherein said material in said accumulator is heatedby passing an electric current through said material directly or throughsaid body.
 20. The method defined in claim 18 wherein said material isheated inductively upon removal from said body.
 21. The method definedin claim 18 wherein said material is heated to an evaporation pointthereof in a low pressure mode of operation of said accumulator.
 22. Themethod defined in claim 18 wherein said material is heated to atemperature in excess of its vaporization point in a high temperaturemode of operation of said accumulator.